ذٰلِكَ بِاَنَّهُمۡ اٰمَنُوۡا ثُمَّ كَفَرُوۡا فَطُبِعَ عَلٰى قُلُوۡبِهِمۡ فَهُمۡ لَا يَفۡقَهُوۡنَ
The reason is that they believed (by the word of mouth) but remained disbelievers (at heart). So their hearts have been sealed. And they do not understand (anything).